Eating our way up

Extant hominids, which include apes and humans come from a plant eating ancestry. Early hominids lived in a woodland-savannah environment in Africa, with a diet consisting of ripe fruits and other high nutrients plants. Modern primates still follow this diet with plant sources making up 87 to 99% of their daily intake. In term of basic gut anatomy there are some differences: for example, the greatest gut volume in humans is in the small intestine, in apes it is the colon. The small intestine is the site of nutrient digestion and absorption, its larger size is indicative of a high quality diet, whereas the colon deals with low quality plant sources.

When the environment transition to savannah-type around 2.5 Ma (million years ago), high quality plant materials such as fruits became difficult to procure. Of the early hominids – Paranthropus (or robust Australopithecus, Homo’s cousin genus) turned to lower quality plant food such as potatoes. The diet is seen today in gorillas and orangutans, and research has shown that a low plant quality diet, which lack of extra energy and nutrients, is linked to social regression and diminution of mobility. Hence compared to the other hominins, orangutans and gorilla are slower, larger and prefer to live alone.

When forests became increasingly scarce it become increasingly taxing to acquire the same diet food source than before. The lack of food results in increased size of the small intestine which controls nutrients’ digestion and absorption, and decreased the size of colon in the other early hominids.

The not-at-all-confusing nomenclature of the apes

Whilst gracile Australopithecus, another cousin genus of Homo, managed to maintain the same diet in the changing environment, Homo had turned to animal sourced food (ASF), i.e. meat. Primates have the ability to digest animal sourced food, which are difficult to procure in forest canopy. Anthropological and archaeological evidence suggested that the earliest Homos were scavengers. This is supported by fossilized bone with fracture marks made by stone tools at 2.5 Ma sites.

ASF hold a significantly higher amount of nutrients and vitamins than the same amount of plant materials, which is crucial in the development of the brain, an organ which requires large amounts of energy and nutrients. With the source of nutrients solved, energy requirement can be satisfied by lower quality plant material that would otherwise be unacceptable, making survival much easier. This is likely to be the main reason Homo continued to thrive whilst hominins like Australopithecus became extinct by 1.2 Ma.

The important dietary changes can be noted by the decrease in tooth size in Homo ergaster compared to Australopithecus and earlier Homos. Larger teeth are indicative of grinding of vegetation, and the decrease in tooth size suggests an evolutionary direction where food preparation had occurred to aid digestion, and meat has become a staple in their diet making large teeth unnecessary. With the increasing cranial capacity and use of stone tools, early Homos were able to hunt animals for food, which further increased their ASF intake.

Another important food source are fish and shellfish, which provide polyunsaturated fatty acids that are vital for brain development. Although signs of aquatic animal consumption can be traced as far back as 1.8 Ma, it wasn’t a main part of the diet until Homo sapiens. This shift in diet is likely a result of the severe competition between the hominoids for remaining food resources brought about by the disappearance of forests. Freshwater fish and shell fish, which were abundant in East African Rift lakes where many early human settlements were found, are rich in lipids: a basic organic molecule very similar to those made up the human brains and are crucial in the brain development.

Archeological evidence shows that Homo sapiens emerged out of Africa around 100,000 years ago via a coastal route dispersing throughout the Old World (which excludes Americas and Australia). They processed basic raft building and navigation skills on the sea and were able to make their way to Australia 35 thousand years ago. They also show signs of cultural evolution, including more complex tools, art, music and languages not seen in previous Homo, all because our ancestor chose the right diet.

Yung Nam Cheah is studying for an MSci in Geology; A shortened version of this is printed in Issue 30 (Spring 2015)